1. Field of the Invention
The present invention relates to antenna systems. More particularly, the present invention relates to antenna systems that are particularly useful for electromagnetic compatibility (EMC) testing.
2. The Prior Art
EMC testing systems are known in the art. EMC testing is used for radiation and susceptibility testing of electronic devices. Radiation testing is used to determine how much RF energy is radiated by electronic devices while they are operating. In the radiation mode of testing, the antenna is used as a receive antenna. Susceptibility testing is used to determine how much RF energy a device can be exposed to before it malfunctions. In the susceptibility mode of testing, the antenna is used as a transmitting antenna.
Susceptibility testing starts at 30 MHz and at that frequency a very large shielded room is required to accommodate a reasonably efficient antenna such as a half wave dipole, which is 16 feet wide at this frequency. These rooms are not practical for the vast majority of test facilities. In addition, because the testing is performed at frequencies up to at least several hundred megahertz, the presently used antenna systems must be broadbanded enough to operate over these ranges.
Presently-available broadband EMC antennas used for susceptibility testing include bi-conical antennas and log periodic arrays. Such antennas are inefficient and can also be expensive. These antennas are paired with RF amplifiers in EMC systems the total cost of which can exceed $500K to $700K. Because presently-available systems are so inefficient, the amplifiers have to be run at extremely high power levels (e.g., in excess of 5 KW). At such power levels, coupled with extreme impedance mismatches between the amplifier and antenna, the EMC antenna sometimes radiates more energy at the second or third harmonic than at the fundamental frequency of interest. This setup badly skews the test results and often renders them invalid according to the existing testing standards.
Other antennas such as e-field radiators, also known as billboard antennas, are sometimes used for susceptibility testing but cannot totally illuminate the device under test. These antennas are thus very inefficient because the test must be repeated. In addition, the test cannot always be performed according to the existing testing standards.
There currently exists a need in the EMC industry for EMC test systems that can generate very high E fields to facilitate susceptibility testing and that conform to new stricter standards which specify that little or no harmonic energy be present. As previously noted, presently-available antenna/amplifier combinations require impractical power levels and require moving the antenna many times at every frequency test point to cover the required illumination of the Equipment Under Test (EUT). This requires many additional hours to perform the test. Additionally, using the presently-available antenna/amplifier combinations results in radiating energy at harmonic frequencies at power levels often exceeding the desired test frequency.
Adjustable antennas using flexible elements currently known in the art have attributes that could make them viable candidates for solving these long known problems in EMC susceptibility testing, but their basic design severely limits their upper frequency limit. Testing standards, such as MIL-STD-461F, require 200V/meter fields, with the antenna one meter away, from 30 MHz to 200 MHz, which is very difficult to achieve with available equipment. Current adjustable-frequency antenna designs, such as those disclosed and claimed in U.S. Pat. No. RE42,087, employ stepper motors to drive copper tape from reels. The reels are in close proximity to each other, resulting stray capacitive and inductive coupling between the element halves that is substantial within the frequency range of EMC testing systems. This limits current designs to an upper frequency limit of about 55 MHz.
In addition, the use of brushes contacting the metal tape to couple the RF energy to and from the tape does not allow the RF to be transferred directly to the starting point of each element resulting in the inadvertent formation of an RF “stub” that can make the system inoperable at various frequencies in the 30 MHz to 200 MHz range.
A second deficiency of current adjustable antennas using elements that loop back on themselves to shorten the width of the antenna is that many shielded rooms are still not large enough to accommodate them even with the 40% shorter elements these antennas provide.
Shielded rooms cause a multitude of “room effects” that severely affect the input impedance of the antenna resulting in very high standing waves that put a tremendous strain on the amplifier, but more importantly cause the harmonic output of the amplifier to be greatly increased.